1. Field of the invention
The present invention relates to an illumination optical system configured to illuminate a surface with a light flux from a light source unit. More specifically, the present invention relates to a projection-type image display apparatus (a liquid crystal projector, for example) configured to illuminate an image display element such as a liquid crystal panel provided on an illuminated surface, using the illumination optical system, and to project light from the image display element onto a projection surface such as a screen.
2. Description of the Related Art
Various types of conventional projectors use a projection lens to magnify and project onto a screen a light flux that has been modulated according to image information using an image display element such as a liquid crystal display element. With respect to a projector like this, it is strongly desired that an image projected on a screen has a nearly uniform brightness over the entire image.
A conventional image projection-type illumination optical system (that uses a reflection type liquid crystal display element) emits a light flux from its light source unit as a substantially parallel light reflected by a parabolic reflector. The parallel light flux is split and condensed in a first lens array. Each split light flux is condensed in the proximity of a second lens array to form a light source unit image (secondary light source image). A lens that constitutes each lens array has a rectangular shape similar to a liquid crystal panel, which is an illuminated surface.
A plurality of split light fluxes that has exited from the second lens array is condensed by a condenser lens and is superimposed on a liquid crystal display element via a color separating/combining optical system to illuminate the liquid crystal display element.
An optical element (a dichroic prism or a polarizing beam splitter) including a dichroic film and a polarizing splitting film is used in a color separating/combining optical system.
In order to increase the light utilization efficiency in an illumination optical system like this, an angular distribution of a light flux which is incident on an illuminated surface generally becomes large. Accordingly, various kinds of problems may occur in the case where an optical element having a sensitive angular characteristic is used in an illumination optical system.
For example, an optical element such as a polarizing splitting film (a polarizing beam splitter) may be used in an illumination optical system. When a light flux is incident on a polarizing splitting film, an angular characteristic of the optical element receiving the light flux incident on the polarizing splitting film having an incidence angle of 47 to 49 degrees is lower than the optical element receiving a light flux incident on the polarizing splitting film having an angle of 45 degrees. As a deviation of an incidence angle from the particular angle of 45 degrees becomes larger, the characteristic gets more severely degraded. The degradation of the characteristic may cause the phenomenon of leaking light, which may cause degradation of image quality.
Japanese Patent Application Laid-Open No. 2000-206463 and Japanese Patent Application Laid-Open No. 06-75200 discuss an illumination optical system that uses an asymmetrical optical element whose angular distribution is relatively small in a direction in which the optical element is sensitive to the angular distribution and relatively large in a direction in which the optical element is insensitive to the angular distribution.
In a reflection type liquid crystal projector as discussed in Japanese Patent Application Laid-Open No. 2000-206463, an outer dimension of an aperture stop is smaller in a direction in which an optical element is sensitive to an angular distribution of a light flux and larger in a direction in which an optical element is insensitive to an angular distribution of a light flux.
In the illumination optical system discussed in Japanese Patent Application Laid-Open No. 2000-206463, a second fly-eye lens provided in an illumination optical system has an asymmetrical outer shape (dimension) because the second fly-eye lens substantially functions as an aperture stop. In such a second fly-eye lens, the angular distribution of a light flux incident on the optical element is asymmetrical with respect to a direction sensitive to the angular distribution and a direction insensitive to the angular distribution
However, in the optical system discussed in Japanese Patent Application Laid-Open No. 2000-206463, a large amount of brightness tends to be lost because an angle of incidence is determined according to a stop.
In an illumination optical system discussed in Japanese Patent Application Laid-Open No. 06-75200, a condenser lens condenses a light flux immediately before a liquid crystal panel. Accordingly, the degradation of image quality cannot be prevented, which may occur due to an optical element disposed behind the condenser lens that has a high sensitivity to an angular distribution, such as the liquid crystal panel or a dichroic mirror. Consequently, the distribution of illuminated light flux on the liquid crystal panel may vary, which may bring about an uneven brightness of an image projected onto a projection screen.